This invention relates to an antilock braking system and more particularly to such a system providing for filtering of the control modes selected in response to wheel operating parameters.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the wheel surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value.
An antilock control system achieves this objective by detecting an incipient lockup condition of the wheel. Upon detecting such a condition, the antilock control system releases pressure at the wheel brake to allow recovery from the incipient lockup condition. Upon recovery, brake pressure is reapplied to the wheel. This cycle is rapidly repeated until the vehicle stops or until the operator applied braking pressure is reduced to a level below that which produces an incipient wheel lockup condition.
The release and apply phases of brake pressure control during antilock controlled braking may each provide for variable rates of change in brake pressure as a function of selected wheel parameters such as wheel slip and acceleration. For example, the reapply phase of brake pressure control following recovery from an incipient wheel lockup condition may include a plurality of brake pressure control modes stored in a lookup table each having a pressure ramp rate associated therewith. The modes are stored as a function of selected wheel parameters such as wheel slip and wheel acceleration to provide a schedule of sequential ramp rates and are used to establish the desired pressure ramp rate corresponding to the specific combination of values of those parameters. The table of brake pressure control modes may provide for progressively decreasing pressure ramp rates as the wheel approaches an incipient wheel lockup condition as represented by increasing wheel slip and decreasing wheel acceleration (increasing wheel deceleration).
In this form of antilock controlled braking having a plurality of ramp rates for the apply and/or release phases of pressure modulation, as a result of factors such as wheel speed noise associated with road surface irregularities, etc., there is a potential for irregular transitions between the ramp rates resulting in large momentary shifts in the brake pressure ramp rate. This shift is undesirable particularly at low ramp rates that are generally associated with approaching critical slip where it is undesirable to overshoot the pressure establishing the critical slip value and therefore the maximum braking effort.